Vortex Dynamics and Instabilities in Tax Ge1-x/Ge Multilayers

Abstract:

In this thesis the magnetic response of a layered type-II superconducting system is explored across the entire range of fields, temperatures and currents where superconductivity exists, with the results providing valuable insight into the role of reduced dimensionality in determining the behaviour of type-II materials such as the new high temperature superconductors. The system in question consists of alternating layers of amorphous Ta or TaxGe1-x (x approximation 0.3) with amorphous Ge where the individual layer thicknesses vary between 17A [angstrom] and 210A [angstrom]. These multilayers were fabricated by vapour deposition in a high vacuum chamber which allowed the creation of samples with uniform layers of high purity. The resistive transport properties have been measured from Tc (approximation 1-3K) to temperatures as low as 50mK in some cases, and in fields of up to 15T. The upper critical fields have been determined from the fluctuation conductivity both with the field parallel and perpendicular to the layer plane of the samples. The results show clearly the dependence of the dimensionality on the superconducting layer thickness and the degree of coupling across the Ge layers. For the samples with the most two-dimensional properties the zero field resistive transition is governed by the unbinding of thermally created vortex-antivortex pairs as described by the Berezinskii-Kosterlitz-Thouless theory. A detailed investigation of the perpendicular field vortex states and dynamics has been performed, including measurement of the activation energies needed for thermally activated vortex motion. Qualitative difference are observed between the activation energies in two- and three-dimensional samples, with the barriers being generally higher in 3D. The non-linear current-voltage characteristics of the samples provide evidence for the existence of a vortex glass state which melts into a liquid below Hc2, although the divergence of the activation barriers in the glass can be restricted by the finite sample thickness. A brief investigation of the corresponding parallel field regime showed considerably less dissipation, due largely to the transparent nature of the Ge layers to the magnetic field. At the highest currents an instability is observed in the vortices which can drive the samples discontinuously back into the normal state. This instability is shown to be of the type predicted by Larkin and Ovchinnikov (LO), including quantitative agreement between the measured and predicted values of the critical vortex velocity. Several features of the instability are noted which are not specifically predicted by the LO theory, and comparisons are drawn between these and the prevailing vortex state at lower currents.